The present invention relates to a composition for developing photosensitive coatings. More specifically, the present invention relates to a method of developing photosensitive lithographic printing plates and similar photographic elements. More particularly, the invention relates to aqueous alkaline developers suitable for removing the non-image areas of negative working photographic elements, which developers are free of organic solvents.
The art of lithographic printing depends upon the immiscibility of grease and water, upon the preferential retention of a greasy image-forming substance by an image area, and upon the similar retention of an aqueous dampening fluid by a non-image area. When a greasy image is imprinted upon a suitable surface and the entire surface is then moistened with an aqueous solution, the image area will repel the water and the non-image area will retain the water. Upon subsequent application of greasy ink, the image portion retains ink whereas the moistened non-image area repels it. The ink on the image area is then transferred to the surface of a material on which the image is to be reproduced, such as paper, cloth and the like, via an intermediary, a so-called offset or blanket cylinder, which is necessary to prevent mirror-image printing.
The most common type of lithographic plate to which the present invention is directed has a coating of a light-sensitive substance that is adherent to an aluminum base sheet. Depending upon the nature of the photosensitive coating employed, the treated plate may be utilized to reproduce directly the image to which it is exposed, in which case it is termed a positive acting plate, or to produce an image complementary to the one to which it is exposed, in which case it is termed a negative acting plate. In either case the image area of the developed plate is oleophilic and the non-image is hydrophilic.
Exposure is effected through a negative transparency, where the light sensitive material, commonly a diazo compound, is caused to harden and thereby become insoluble in a desensitizing solution applied to the plates after light exposure for the purpose of removing that part of the light sensitive coating which, because it was protected from the light by the negative, was not light hardened. The light hardened surface of a negative plate will be the oleophilic surface compatible with the greasy ink and is called the "image-area". The surface from which the non-hardened light sensitive material has been removed by a desensitizer will be, or can be, converted to a hydrophilic surface having little affinity for the greasy ink and is called the "non-image" area.
The present invention provides a new developer for negative working lithograpic printing plates.
Most developing compositions for negative working printing plates disadvantageously contain strong organic solvents. These are both expensive and ecologically not favored. In recent years certain aqueous alkaline developing compositions have been developed and employed for such purposes. While they are less costly and dangerous to the environment, they do pose several technical problems. Specifically, they tend to corrode aluminum, foam and precipitate when used in developing machinery and additionally have a relatively slow development speed. The present invention provides a developing composition which substantially alleviates these problems.